Low loss high-q filter

ABSTRACT

A low loss high-Q filter is disclosed herein having a restricted bandpass of 7.25-7.75 GHz with an in-band insertion loss of 0.5db, an out-of-band rejection greater than 80db and negligible spurious responses in-band. This is accomplished by providing two types of directly coupled variable resonators. The first type of variable resonators includes a series of twelve right circular cylindrical cavity resonators directly coupled in series with each other. Two of the second type of variable resonators are provided with each of these second type of variable resonators being directly coupled to opposite ends of the series coupled cylindrical resonators. Each of these second type of variable resonators include three directly coupled rectangular waveguide cavity resonators.

[ 11 3,845,423 [451 Oct. 29, 1974 United States Patent [191 Scheiner LOW LOSS HIGH-Q FILTER Primar Examiner.lames W. Lawrence c t ,N.Y. y [75] inventor Harry Schemer New 1 y Assistant Examiner-Marvm Nussbaum Assigneei lfltel'nafimlfll Telephone and Attorney, Agent, or Firm-John T. OI-lalloran;

Telegraph Corporatwn, Nutley, J. Edward Goldberg; Alfred 0. Hill Sept. 26, 1973 21 Appl. No.: 400,775

[22] Filed:

ABSTRACT A low loss high-Q filter is disclosed herein having a ref variable resonators are provided with each of these second type of variable resona stricted bandpass of 7.25-7.75 Gl-lz with an in-band insertion loss of 0.5db, an out-of-band rejection greater than 80db and negligible spu in-band. This is accomplished by providing two types of directly coupled variable resonators. The first type of variable resonators includes a series of twelve right circular cylindrical cavity in series with each other. Two of the second type 0 pled to opposite ends of the series coupled cylindrical resonators. Each of these second type of variable reso- RMR XWRWX RY oo R3%3R 9 9 7 7 D. one M8 /%3 3 3 3 3 n a B N m .L 3 m d m m m M mm W 3 .I. m W S mt n mm. B m w v m CT mflmas a ma m m w "U T aluSU 3 .mSVKGTL U. 8 "we D m fl E34 e T55777 [99999 .n Nlllll O f Ill/l rL U7 362 C 1 l UmF 0860 2 .7 4417 HUN 6 mwmw 555. "D 2 2 3 3 3 OTHER PUBLICATIONS De Mornay.Budd Standard Microwave 2nd Edition, (received in US. Patent Ofnators include three directly coupled rectangular waveguide cavity resonators.

Catalog, Equipment fice June 14, 1948), cover page and pages l516. 18 Clams 3 Drawmg Figures PMENIEB URI! 2 9 W4 i ii LOW LOSS HIGH-Q FILTER BACKGROUND OF THE INVENTION This invention relates to a low loss high-O filter and more particularly to such a filter that can be employed in communication systems as the receiver preselector filter.

Adjacent frequency bands have often been used in communication systems for simultaneous transmission and reception of intelligence. The commonality of design afforded by this choice of system is obvious, however, duplex operation has become more difficult as a result of imrovements in power amplifiers and receivers. Transmitters producing kilowatts in a frequency band adjacent to the operating band of a low noise receiver impose stringent requirements on the receiver preselector filter. The preselector filter must reject the transmitted energy to prevent degradation of the receiver and at the same time be sufficiently low loss so that the contribution to the system temperature is minimal. The figure of merit used to evaluate the performance of a communication terminal is the system gain vs. the temperature ratio. An increase in temperature of a receiver obviously reduces the figure of merit, but the insertion loss of the preselector filter has a dual effect (I) it reduces gain and (2) increases noise temperature. An increase in temperature is approximately 7 for each 0. ldb (decibel) increase in insertion loss where the total insertion loss is less than l.0db and,

conversely, every 0.1db improvement obtained in the preselector filter is equivalent to a 7 improvement in the receiver for the same combined noise temperature.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a preselector low loss high-Q filter which can satisfy the requirements imposed by present day communication systems.

Another object of the present invention is to provide a preselector high-Q filter that will provide a lower inband loss than conventional waveguide filters.

Still another object of the present invention is to provide a low loss high-Q preselector filter that can meet the requirements of present day communication systems and can be readily manufactured.

A feature of the present invention is the provision of a low loss high-Q filter comprising: a plurality of right circular cylindrical cavity resonators; a first plurality of rectangular waveguide cavity resonators connected to one end of the cylindrical resonators; a second plurality of rectangular waveguide cavity resonators connected to the other end of the cylindrical resonators; first means for direct coupling of the cylindrical resonators in series and for direct coupling to the first and second waveguide resonators; and second means for direct coupling of each of the first and second plurality of waveguide resonators in series.

BRIEF DESCRIPTION OF THE DRAWING Above-mentioned and other features and objects of this invention will become more apparent by reference to the following description taken in conjunction with the accompanying drawing, in which:

FIG. 1 is a side elevational view of a low loss high-Q filter in accordance with the principles of the present invention with the tuning arrangements of the right circular cylindrical cavity resonators omitted for clarity;

FIG. 2 is a top view of the low loss high-O filter of FIG. 1 with the tops of the right circular cylindrical cavity resonators removed; and

FIG. 3 is a longitudinal cross sectional view of the low loss high-Q filter of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT In the following description an actual reduction to practice of the low loss high-O filter of the present invention will be described. The filter of the reduction to practice had the following characteristics: a passband of 7.25 7.75 GHz (gigahertz), an in-band insertion loss of 0.5 db, a passband standing wave ratio of 1.2, a Q in the order of 10,000 as indicated by the measured passband insertion loss and an out-of-band rejection greater than db. The various dimensions that are given hereinbelow are for this actually reduced to practice filter. However, it should be noted that the low loss high-Q filter of this invention may be dimensioned to have other passbands, in-band insertion losses, values of Q, passband standing wave ratios and out-of-band rejection characteristics.

Referring to FIGS. 1, 2 and 3, there is illustrated therein a reduction to practice of a low loss high-Q filter in accordance with the principles of the present invention. The filter of the reduction to practice of the present invention is an eighteen pole, 0.0ldb ripple, Chebyshev filter including a central filter section I having formed therein a first type of variable cavity resonators and two identical end filter sections 2 and 3 having formed therein a second type of variable cavity resonators. Filter section 2 is directly coupled to one of the end cavity resonators of filter section 1 and filter section 3 is directly coupled to the other of the end cavity resonators of filter section 1.

Filter section 1 includes twelve right circular cylindrical cavity resonators 4-15 with each of these cylindrical resonators being directly coupled to each other by means of circular apertures 16-26 disposed adjacent the bottom of each of cylindrical resonators 4-15. Resonator 4 is provided with a circular aperture 27 to provide direct coupling to filter section 2 while resonator 15 is provided with a circular aperture 28 to provide direct coupling to filter section 3. Each of the apertures 16-28 are at right angles with respect to the longitudinal axis of each of cylindrical resonators 4-15 and extend through the cylindrical wall of these cylindrical resonators.

It should be noted from the illustration that filter section 1 is actually formed in three parts, a central part 29 and two identical end parts 30 and 31. The central part 29 includes six whole cylindrical resonators 7-12, a half of cylindrical resonator 6 and a half of cylindrical resonator 13. End part 30 includes therein whole cylindrical resonators 4 and 5 and end part 31 includes therein whole cylindrical resonators 14 and 15. End part 30 includes the other half of resonator 6 while end part 31 includes the other half of cylindrical resonator 13. The parts 29-31 are appropriately connected together to form filter section I such as by providing flanges 32 and 33 to connect end part 30 to central part 29 by employing a suitable fastener, such as bolts 34 and nuts 35. Similarly, flanges 36 and 37 are provided on parts 29 and 31, respectively, which are suitably fastened together, for instance, by bolts 38 and nuts 39.

Each of the resonators 4-15 include therein a tuning arrangement such as a choke coupled double bucket plunger 40-51. Each of these plungers 40-51 include two buckets 52 and 53 tapped to receive tuning bolt 54. Buckets 52 and 53 are dimensioned and spaced from the inner cylindrical surface of the cylindrical resonators an amount appropriate for the passband frequency involved. Each of the plunger units 40-51 include a plastic disc 55 which is tapped to receive tuning screw 54. Disc 55 contacts the inner surface of each of the cylindrical resonators to enable the plunger units 40-51 to be smoothly guided up and down the cylindrical resonators coaxial of the longitudinal axis of these cylindrical resonators. A locking nut 56 is provided in association with the tuning bolt 54 and the top 57 to lock the plunger units 40-51 in the position that properly tunes the associated cylindrical resonator. Each of the tuning plungers 40-51 have the same construction and relationship to their associated cylindrical cavity and, therefore, each of these plunger units will not be described separately.

For the reduction to practice, each of the cylindrical resonators 4-15 had a diameter A equal to 1.300 inches and a depth B equal to 2.875 inches. The spacing C between adjacent cylindrical resonators was equal to 0.062 inches. Each of the circular coupling apertures 16-28 had a diameter D equal to 0.555 inches with the center of these coupling apertures being disposed a distance E equal to 0.410 inches from the bottom surface of each of the cylindrical resonators. The width F of the block in which the cylindrical resonators are formed was equal to 1.500 inches.

Each of the filter sections 2 and 3 are identical as mentioned hereinabove and are secured to the ends of filter section 1 by means of bolts 58 and 59 and nuts 64 and 65. Bolts 58 and 59 are secured to flanges 60 and 61 and extending through apertures in flanges 62 and 63.

Each of the filter sections 2 and 3 are formed from a rectangular waveguide having inner dimensions of G X H which is equal to 1.252 X 0.628 inches. Each of the waveguide sections 66 and 67 include therein three rectangular waveguide resonators 68, 69 and 70 with irises 71, 72 and 73 defining the rectangular waveguide cavity resonators and providing the direct coupling be-' tween these resonators. Capacitive tuning bolts 74, 75 and 76 tune the associated rectangular waveguide resonators 68, 69 and 70. Nuts 77, 78 and 79 are locking nuts for capacitive tuning bolts 74, 75 and 76, respectively.

1n the actual reduction to practice, waveguides 66 and 67 had a length 1 equal to 3.313 inches while resonator 68 had a length J equal to 0.915 inches, resonator 69 had a length K equal to 0.935 inches and resonator 70 had a length L equal to 0.838 inches. The distance M between iris 71 and the output of the waveguide 66 had a value equal to 0.625 inches. Iris 71 had an iris opening N equal to 0.506 inches, iris 72 had an iris opening equal to 0.281 inches and iris 73 had an iris opening P equal to 0.236 inches. These dimensions given for the iris openings are symmetrical with the longitudinal center line of the waveguides 66 and 67. Each of the iris vanes forming the iris openings have a width Q equal to 0.032 inches. The capacitive tuning bolt 74 was spaced from the center line of iris 71 an R equal to 0.419 inches, capacitive tuning bolt 75 was spaced from the center line of iris 72 an amount S equal to 0.467 inches and capacitive tuning bolt 76 was spaced from the center line of iris 73 an amount T equal to 0.473 inches.

Again in accordance with the reduction to practice described herein the waveguides 66 and 67 were copper rectangular waveguides while the cylindrical resonators 4-15 were formed in a block of aluminum. When the aluminum filter section 1 and the copper waveguides 66-67 were fastened together and tested it was found that the filter had an in-band insertion loss of 1.0 db. This insertion loss was not the insertion loss required and, therefore, the filter was disassembled and the aluminum material forming filter section 1 had the inner surfaces of the cylindrical resonators 4-15 silver plated. The filter was then assembled and again tested and it was discovered that the filter now had the desired in-band insertion loss of 0.5 db.

The low loss high-Q filter described with respect to FIGS. 1, 2 and 3 was built by selecting for filter section 1 a resonator building block of the right circular cylindrical type, first, because of the higher 0 available with this type of resonator as compared with the standard waveguide resonator and, secondly, for ease of fabrication as comparedwith an elliptical or spherical resonator. The filter in accordance with the principles of the present invention uses two different resonators directly coupled within the same filter structure in which internal coupling values are satisfied to provide the desired in-band response and out-of-band rejection and, thus, provides an efficient preselector filter for a receiver that can be employed in present-day communication systems.

While 1 have described above the principles of my invention in connection with specific apparatus it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of my invention as set forth in the objects thereof and in the accompanying claims.

1 claim:

1. A low loss high-Q filter comprising:

a plurality of right circular cylindrical cavity resonators;

a first plurality of rectangular waveguide cavity resonators connected to one end of said cylindrical resonators;

a second plurality of rectangular waveguide cavity resonators connected to the other end of said cylindrical resonators;

first means for direct coupling of said cylindrical resonators in series and for direct coupling to said first and second waveguide resonators; and

second means for direct coupling of each of said first and second plurality of waveguide resonators in series;

each of said cylindrical resonators having a longitudinal axis and each of said cylindrical resonators being oriented with respect to each other so that each of said longitudinal axes are parallel with respect to each other;

said first means including a plurality of circular apertures, each of said plurality of apertures being disposed adjacent the bottom of an associated one of said cylindrical resonator, at right angles to said longitudinal axis of said associated one of said cylindrical resonators and to extend through the cylindrical wall of said associated one of said cylindrical resonators to provide direct coupling between adjacent ones of said cylindrical resonators, and

pair of circular apertures, one of said pair of circular apertures being disposed adjacent the bottom of a first of said cylindrical resonators, at

right angles to said longitudinal axis of said first of said cylindrical resonators and to extend through the cylindrical wall of said first of said cylindrical resonators to provide direct coupling to said first plurality of waveguide resonators,

and the other of said pair of circular apertures being disposed adjacent the bottom of a last of said cylindrical resonators, at right angles to said longitudinal axis of said last of said cylindrical resonators and to extend through the cylindrical wall of said last of said cylindrical resonators to provide direct coupling to said second plurality of waveguide resonators.

2. A filter according to claim 1, wherein each of said first and second plurality of waveguide resonators are iris coupled.

3. A filter according to claim 1, wherein said cylindrical resonators are formed in an aluminum block and the inner surfaces of said cylindrical resonators are silver plated.

4. A filter according to claim 2, further including a plurality of tuning means, each of said plurality of tuning means being disposed in a different one of said cylindrical resonators coaxial of said longitudinal axis.

5. A filter according to claim 2, further including a plurality of tuning means, each of said tuning means being disposed in a different one of said first and second plurality of waveguide resonators.

6. A filter according to claim 2, wherein each of said irises are oriented at right angles with respect to said longitudinal axes.

7. A filter according to claim 2, further including a first plurality of tuning means, each of said first plurality of tuning means being disposed in a different one of said cylindrical resonators coaxial of said longitudinal axis; and

a second'plurality of tuning means, each of said second plurality of tuning means being disposed in a different one' of said first and second plurality of waveguide resonators.

8. A filter according to claim 4, wherein each of said plurality of tuning means includes a choke coupled double plunger.

9. A filter according to claim 5, wherein each of said plurality of tuning means includes a capacitive tuning screw.

10. A filter according to claim 7, wherein each of said first plurality of tuning means includes a choke coupled double plunger; and

each of said second plurality of tuning means includes a capacitive tuning screw.

11. A filter according to claim 10, wherein said cylindrical resonators are formed in an aluminum block and the inner surfaces of said cylindrical resonators are silver plated.

12. A filter according to claim 11, wherein said first and second waveguide resonators are formed in a rectangular copper waveguide.

13. A filter according to claim 3, wherein said first and second waveguide resonators are formed in a rectangular copper waveguide.

14. A filter according to claim 13, wherein each of said first and second plurality of waveguide resonators are iris coupled.

15. A filter according to claim 14, wherein each of said irises are oriented at right angles with respect to said longitudinal axes.

16. A low loss high-Q filter comprising:

a plurality of right circular cylindrical cavity resonators;

a first plurality of rectangular waveguide cavity resonators connected to one end of said cylindrical resonators;

a second plurality of rectangular waveguide cavity resonators connected to the other end of said cylindrical resonators;

first means for direct coupling of said cylindrical resonators in series and for direct coupling to said first and second waveguide resonators; and

second means for direct coupling of each of said first and second plurality of waveguide resonators in se ries;

said filter being an eighteen pole, 0.0l decibel ripple, Chebyshev filter with said cylindrical resonators equal to 12 and each of said first and second waveguide resonators equal to 3.

17. A filter according to claim 16, wherein each of said cylindrical resonators are formed in an aluminum block and the inner surfaces of said cylindrical resonators are silver plated; and

said first and second waveguide resonators are formed in a rectangular copper waveguide.

18. A filter according to claim 17, wherein said aluminum block includes two end portions and a central portion, said end portion and said central portion being joinable to form said twelve cylindrical resonators,

each of said end portions having two and a half cylindrical resonators of said 12 cylindrical resonators, and

said central portion having a first half cylindrical resonator of said twelve cylindrical resonators to join with said half cylindrical resonator of one of said end portion to form a full cylindrical resonator of said 12 resonators, a second half cylindrical resonator of said 12 cylindrical resonator to join with said half cylindrical resonator of the other of said end portion to form a full cylindrical resonator of said twelve cylindrical resonators and six full cylindrical resonators of said twelve cylindrical resonators disposed in series between said first and second half cylindrical resonator. 

1. A low loss high-Q filter comprising: a plurality of right circular cylindrical cavity resonators; a first plurality of rectangular waveguide cavity resonators connected to one end of said cylindrical resonators; a second plurality of rectangular waveguide cavity resonators connected to the other end of said cylindrical resonators; first means for direct coupling of said cylindrical resonators in series and for direct coupling to said first and second waveguide resonators; and second means for direct coupling of each of said first and second plurality of waveguide resonators in series; each of said cylindrical resonators having a longitudinal axis and each of said cylindrical resonators being oriented with respect to each other so that each of said longitudinal axes are parallel with respect to each other; said first means including a plurality of circular apertures, each of said plurality of apertures being disposed adjacent the bottom of an associated one of said cylindrical resonator, at right angles to said longitudinal axis of said associated one of said cylindrical resonators and to extend through the cylindrical wall of said associated one of said cylindrical resonators to provide direct coupling between adjacent ones of said cylindrical resonators, and a pair of circular apertures, one of said pair of circular apertures being disposed adjacent the bottom of a first of said cylindrical resonators, at right angles to said longitudinal axis of said first of said cylindrical resonators and to extend through the cylindrical wall of said first of said cylindrical resonators to provide direct coupling to said first plurality of waveguide resonators, and the other of said pair of circular apertures being disposed adjacent the bottom of a last of said cylindrical resonators, at right angles to said longitudinal axis of said last of said cylindricaL resonators and to extend through the cylindrical wall of said last of said cylindrical resonators to provide direct coupling to said second plurality of waveguide resonators.
 2. A filter according to claim 1, wherein each of said first and second plurality of waveguide resonators are iris coupled.
 3. A filter according to claim 1, wherein said cylindrical resonators are formed in an aluminum block and the inner surfaces of said cylindrical resonators are silver plated.
 4. A filter according to claim 2, further including a plurality of tuning means, each of said plurality of tuning means being disposed in a different one of said cylindrical resonators coaxial of said longitudinal axis.
 5. A filter according to claim 2, further including a plurality of tuning means, each of said tuning means being disposed in a different one of said first and second plurality of waveguide resonators.
 6. A filter according to claim 2, wherein each of said irises are oriented at right angles with respect to said longitudinal axes.
 7. A filter according to claim 2, further including a first plurality of tuning means, each of said first plurality of tuning means being disposed in a different one of said cylindrical resonators coaxial of said longitudinal axis; and a second plurality of tuning means, each of said second plurality of tuning means being disposed in a different one of said first and second plurality of waveguide resonators.
 8. A filter according to claim 4, wherein each of said plurality of tuning means includes a choke coupled double plunger.
 9. A filter according to claim 5, wherein each of said plurality of tuning means includes a capacitive tuning screw.
 10. A filter according to claim 7, wherein each of said first plurality of tuning means includes a choke coupled double plunger; and each of said second plurality of tuning means includes a capacitive tuning screw.
 11. A filter according to claim 10, wherein said cylindrical resonators are formed in an aluminum block and the inner surfaces of said cylindrical resonators are silver plated.
 12. A filter according to claim 11, wherein said first and second waveguide resonators are formed in a rectangular copper waveguide.
 13. A filter according to claim 3, wherein said first and second waveguide resonators are formed in a rectangular copper waveguide.
 14. A filter according to claim 13, wherein each of said first and second plurality of waveguide resonators are iris coupled.
 15. A filter according to claim 14, wherein each of said irises are oriented at right angles with respect to said longitudinal axes.
 16. A low loss high-Q filter comprising: a plurality of right circular cylindrical cavity resonators; a first plurality of rectangular waveguide cavity resonators connected to one end of said cylindrical resonators; a second plurality of rectangular waveguide cavity resonators connected to the other end of said cylindrical resonators; first means for direct coupling of said cylindrical resonators in series and for direct coupling to said first and second waveguide resonators; and second means for direct coupling of each of said first and second plurality of waveguide resonators in series; said filter being an eighteen pole, 0.01 decibel ripple, Chebyshev filter with said cylindrical resonators equal to 12 and each of said first and second waveguide resonators equal to
 3. 17. A filter according to claim 16, wherein each of said cylindrical resonators are formed in an aluminum block and the inner surfaces of said cylindrical resonators are silver plated; and said first and second waveguide resonators are formed in a rectangular copper waveguide.
 18. A filter according to claim 17, wherein said aluminum block includes two end portions and a central portion, said end portion and said centrAl portion being joinable to form said twelve cylindrical resonators, each of said end portions having two and a half cylindrical resonators of said 12 cylindrical resonators, and said central portion having a first half cylindrical resonator of said twelve cylindrical resonators to join with said half cylindrical resonator of one of said end portion to form a full cylindrical resonator of said 12 resonators, a second half cylindrical resonator of said 12 cylindrical resonator to join with said half cylindrical resonator of the other of said end portion to form a full cylindrical resonator of said twelve cylindrical resonators and six full cylindrical resonators of said twelve cylindrical resonators disposed in series between said first and second half cylindrical resonator. 